Thermally operated electric switch



Oct. 10, 1950 F. BEST 2,524,954

THERMALLY OPERATED ELECTRIC SWITCH Filed April 25, 1949 umlllllllm v awk/ Patented Oct. 10, 1950 THERMALLY OPERATED ELECTRIC SWITCH Fredric Lafargue Best, Felixstowe, England, as-

signor to Best Products Limited, Felixstowe, England, a British company Application April 25, 1949, Serial No. 89,498 In Great Britain May 3, 1948 (or, 219 ss) 1 Claim.

This invention relates to thermall operated electrical switches and has for its object to provide an improved construction and arrangement which is suitable for various purposes. It may for example be used as a safety-switch in association with an immersion heating element.

In the accompanying drawings, a switch in accordance with thisinvention is illustrated as arranged to constitute a safety switch for an immersion heater. In the drawings:

Figure l is a sectional View showing the im proved safety-switch mounted on an immersion heating element,

Figure 2 is a section on the line 22 of Figure 1, and

Figure 3 is a section on the line 33 of Figure 1.

The heating element of an electric kettle or like vessel is constructed in the form of a loop I of circular or any other desired shape, and comprises an electrical resistance 35 embedded in a refractory insulator 36 in a metal tube, the two ends of the loop H, l2 being secured in a suitable watertight manner in the heater-head l3 which is itself secured to the wall of the kettle. This head [3 is provided with a flang [4 which is located on the inner surface of the wall of the kettle, and the cylindrical part of the head extends through a hole in the wall and is clamped thereto by a nut screwed on to the threaded part [5 on the outside of the wall of the kettle. head carries at its outer end a terminal plate l6 carrying contact pins I! or other suitable means whereto connection is made from the supplymains.

The ends ll, l2 of the heating element adjacent the head l3 are made straight and parallel with one another, and a tubular casing i8 is mounted between them so as to lie in contact or close to each of them. As shown in Figure 3 this casing is of oval external shape with a central cylindrical bore I9, but any other desired shape may be used. It is soldered 0r brazed as shown at 20 to the ends H, l2 of the element throughout its length, so that it is in good thermal contact therewith. This casing l8 also makes a watertight joint with the heater-head l3, and opens into an aperture therein. The other end of the casing at 2| is closed, and has placed in it a ferrule 22 of copper, copper alloy or brass, which is a good conductor of heat, and resilient.

A helix 24 formed from a bi-metallic strip is mounted in the casing l8 and its inner end is anchored thereto by clinching it on the ferrule 22, this clinching being effected by pressure on the outside of the casing in the direction indi- This 3 cated by the arrows 34 in Figure 3. The other, free end of the helix lies near the open end of its casing I8 and a shank is secured therein by a transverse pin 26, this shank extending outwards into the heater-head l3, where it carries an arm or cam-like member 21, see Figure 2.

The connecting-leads for the resistance in the element ID are indicated at 28, 29 respectively and one of them as shown at 29 is connected directly to the terminal pin [1; the lead 28 is connected to a flexible strip 30 which carries one contact member 3| of a switch whereof the other contact member 32 is carried on a bracket 33 which in turn is connected to the second contact pin ll; the arm 21 carried by the helix, and the flexible strip 30, are so disposed and arranged that when the helix is distorted by a rise in temperature, the arm 21 engages the spring 30 in its rotation and separates the contacts 3|, 32 thereby opening the circuit to the heating element Hi. The parts are so proportioned that this opening of the circuit takes place at a temperature slightly above that of boiling water, such as would be obtained if the element were switched on when the kettle is dry or which would be obtained after the water had boiled away sufficiently to expose the element to a risk of damage b overheating.

It will be appreciated that the precise arrangement of the casing for the safety-switch on the heating element may be varied according to size and shape of the heating element, and according to the length of the casing which is required to accommodate the helix, but the essential feature is that the casing should be in good thermal contact with the heating element so that it follows substantially the temperature changes thereof and will become operative before a dangerous temperature is reached.

I claim:

In portable water-heating apparatus the com bination of a liquid-containing vessel, a heaterhead secured on and extending through the wall of the vessel, a pairof contact-pins carried on the outer end of said heater-head, an electric immersion heating element in the form of a loop having straight parallel end-portions which extend into the inner end of said heater-head and thereby support the element, an electric resistor in said heating element, having one end connected to one of said contact-pins, a tubular casing secured in intimate thermal contact with said parallel end-portions of said element, a ferrule within said casing, a helical bimetallic strip within said casing having one end compressed between said casing and said ferrule, a switch-con- FREDRIC LAFARGUE BEST.

REFERENCES CITED The following references are of record in the file of this patent:

Number UNITED STATES PATENTS Name Date Hynes et a1 July 16, 1929 Wells Aug. 1, 1933 I-Iuth Sept. 5, 1933 Blashfield Oct. 17, 1933 Skoverski Oct. 24, 1933 Hodges Oct. 24, 1933 

